Cellular radios require accurate transmit power control to prevent unwanted interference between different wireless networks. This is commonly achieved by a power control loop in a transceiver. The power control loop senses a power signal coupled from a main radio frequency (RF) signal path which is connected to an antenna. This coupled power signal is then fed back to a power detection circuitry through a directional coupler placed between the power detection circuitry and a feed point of the antenna.
The antenna feed point needs to be able to adapt to multiple transmit bands resulting in a wide operating frequency range (e.g., from transmit band 12 at 699 MHz through transmit band 43 at 3.8 GHz in LTE telecommunication) of the transceiver. The transceiver may only be operating at one band at any particular time, but the directional coupler must be capable of operating at any band. It is desirable that the directional coupler is optimized for each operating transmit band.
The insertion loss (IL) of the directional coupler is related to the coupling factor (CF) as shown in equation (1). If the insertion loss needs to stay at a low value (IL<0.15 dB), it is desirable to keep the coupling factor relatively high (CF>20 dB). The insertion loss given by equation (1) can be >0.28 dB with a CF=12 dB.
                              I          ⁢                                          ⁢                      L            coupler                          ≈                              10            ×                          log              ⁡                              [                                  1                  +                                      10                                                                  -                        C                                            ⁢                                                                                          ⁢                                              F                        /                        10                                                                                            ]                                              +                      Implementation            ⁢                                                  ⁢            Losses                                              (        1        )            
In one example, the desired transmit power range of the antenna is from −50 dB to +24 dB and the power detection circuitry has a limited dynamic range, e.g. −78 dBm to +4 dBm. In order to accurately control the transmit power in the desired transmit power range and match the transmit power between the antenna and the power detection circuitry, the coupling factor of the directional coupler needs to be between 28 dB and 20 dB. The dynamic range of the power detection circuitry keeps the same values regardless of the operating frequency; while the coupling factor of the directional coupler may be frequency dependent for some implementations. Typically, the CF requirements at a low operating frequency (e.g. 699 MHz) determine the value of the coupler's coupling factor. At a higher operating frequency, the coupling factor can be compensated by switching in extra attenuation in a feedback path that extends from the directional coupler to the power detection circuitry.
In some applications, the feedback path may include additional switching, filtering and attenuation circuitries. The net coupling factor (CFNet) is therefore the sum of the coupler's native coupling factor (CFNative) and the insertion loss of the feedback path (typically ˜2 dB), as in equation (2).CFNet=CFNative+ILFeedbackPath  (2)The native coupling factor of the directional coupler and the circuitries in the feedback path need to be chosen carefully to keep the net coupling factor in a desirable range.
FIG. 1 illustrates a conventional directional coupler 10 that provides both forward and reverse power detection modes. The combination of the forward and reverse power detection modes provides the ability to estimate the antenna voltage standing wave ratio (VSWR). The coupler 10 includes a main signal path that extends between an RF input port RFin and an RF output port RFout and includes a primary inductive segment L1; a secondary signal path that extends between a coupled port COUPLED and ground and includes a secondary inductive segment L2. The primary inductive segment L1 is mutually coupled with the secondary inductive segment L2. A mutual coupling K12 is very important due to its enablement to keep the inductive segments values low, resulting in a small low loss structure. A coupling capacitance C12 is formed between the primary inductive segment L1 and the secondary inductive segment L2. Switch circuitry (SW1-SW4) is configured to select either forward or reverse power detection mode. When the coupler 10 receives a signal from the RF input port RFin (such as a transmit signal), a coupled signal is detected in the secondary signal path in the forward power detection mode. Herein, the switches SW1 and SW4 are open, the switches SW2 and SW3 are closed, and the secondary inductive segment L2 is coupled between the coupled port COUPLED and a forward isolated port ISOfwd. When the coupler 10 receives a signal from the RF output port RFout (such as a received signal from an antenna), a coupled signal is detected in the secondary signal path in the reverse power detection mode. Herein, the switches SW1 and SW4 are closed, the switches SW2 and SW3 are open, and the secondary inductive segment L2 is coupled between the coupled port COUPLED and a reverse isolated port ISOrev. A first tunable impedance termination ZT1 is used in the forward power detection mode and a second tunable impedance termination ZT2 is used in the reverse power detection mode. The native coupling factor of the coupler 10 has a frequency dependence, as shown in equations (3) and (4), which results in a large change in the coupling factor over the target operating frequency from 699 MHz to 3.8 GHz of around 14.7 dB, as shown in equation (5).
                              C          ⁢                                          ⁢                      F            native                          ≈                              -            20                    ×                      log            (                                          Z                o                                                              Z                  o                                +                                  1                                      i                    ⁢                                                                                  ⁢                    2                    ⁢                    π                    ⁢                                                                                  ⁢                                          F                      o                                        ⁢                                          C                      12                                                                                            )                                              (        3        )            For CF>15 dB CFNative≈Ko−20×log(F0),Ko=−20 log(Zo×2πC12)  (4)
                              Δ          ⁢                                          ⁢          C          ⁢                                          ⁢          F                =                                                            C                ⁢                                                                  ⁢                                  F                                      699                    ⁢                    M                                                              -                              C                ⁢                                                                  ⁢                                  F                                      3.8                    ⁢                                                                                  ⁢                    G                                                                        ≈                          20              ×                              log                ⁡                                  (                                                            3.8                      ⁢                                                                                          ⁢                      G                                                              699                      ⁢                      M                                                        )                                                              =                      14.7            ⁢                                                  ⁢            d            ⁢                                                  ⁢            B                                              (        5        )            Herein, Z0 represents system characteristic impedance, F0 represents the operating frequency, and C12 represents the coupling capacitance between the primary inductive segment L1 and the secondary inductive segment L2. As shown in equation (3), both the operating frequency F0 and the coupling capacitance C12 have an inverse relation with the native coupling factor CFnative. When the operating frequency F0 or the coupling capacitance C12 decrease, the CFnative will increase; when the operating frequency F0 or the coupling capacitance C12 increase, the CFnative will decrease.
The large ΔCF in equation (5) means that even if the CF699 MHZ is at the high end of the desirable range (˜28 dB) at 699 MHz, the CF3.8 GHz at 3.8 GHz will be quite low (˜13.3 dB). Once the feedback path losses of ˜2 dB are also taken into account (e.g., from attenuators, filter switches, and trace loss), the native coupling factor of the coupler at 3.8 GHz could be as low as ˜11.3 dB, resulting in a lot of transmit power being routed to the feedback path and a large insertion loss (˜0.34 dB) with implementation losses.
Furthermore, the size constraints in a modern smartphone require a compact coupler structure and preclude the use of wideband λ/4 transmission line approaches. The insertion loss and size constraints also make coupler structures with multiple stages unattractive.
Therefore, there is a need for a compact directional coupler with a reconfigurable structure, whose coupling factor can be reprogrammed as a function of a desired transmit band of operation and can achieve a desired value in a wide operating frequency range to keep insertion loss low. It is also desirable that the coupler structure is capable of supporting both forward and reverse power detection modes.